Galaxies, Active Galactic Nuclei and Cosmology
PhD Projects
Understanding the relationship between star formation and the Insterstellar Medium in Interacting Galaxies
Elias Brinks, Nicola Brassington, Martin Hardcastle
It is widely believed that very few galaxies exist today that have not been formed or shaped in some way by an interaction with another galaxy. These interactions play a major role in the evolution of galaxies by triggering star formation and nuclear activity. However the parameters that influence the star forming activity in interacting galaxies are poorly understood.
The Spitzer Interacting Galaxies Survey (SIGS), a large international collaboration including researchers in the US and Europe, is addressing this question by using mid-IR data obtained with the Spitzer Space Telescope to study a large sample of 110 galaxies in different stages of interaction. In this project the student will have the opportunity to investigate the relationship between star formation rate (indicated by the mid-IR observations) and the gas surface density, which will be determined through HI and CO observations made with ground-based telescopes. These comparisons will determine if and how the efficiency of turning gas into stars changes as a function of the interaction strength. This is a key parameter in simulations of galaxy mergers and evolution, and therefore these measurements are of great importance for our understanding of the the cosmological evolution of galaxies.
The beauty of LITTLE THINGS
Elias Brinks, Volker Heesen
LITTLE THINGS stands for "Local Irregulars That Trace Luminosity Extremes" and is a successor of the highly successful THINGS or "The HI Nearby Galaxy Survey". Here HI stands for the neutral atomic phase of hydrogen in the interstellar medium (ISM) of galaxies. LITTLE THINGS (LT for short) is an international collaboration of about 15 astronomers, a mix of senior staff, postdoctoral researchers, and PhD students, mainly located in the USA and in Europe. LT is targeting a subset of around 40 galaxies out of a much larger sample of some 120 dwarf irregular galaxies, covering a wide range in mass, luminosity, metallicity, and star formation (SF) activity. Results from THINGS have shown that the SF characteristics of dwarf galaxies resemble that of the outskirts of their bigger brethren, the gas-rich spiral galaxies. THINGS showed that SF depends on the ease at which HI turns molecular. The aim of the proposed PhD project is to extend the study of SF to the realm of dwarf galaxies, probing this process in extreme, low gas density environments, to trace the extent of dwarfs, and map their internal kinematics. More specifically, this project will for the first time take into account the fact that the HI discs of dwarf galaxies are much thicker than those in larger spiral galaxies which means star formation will need to be related to the 3-D gas distribution, rather than surface densities as has been the case until now.
The Interplay of AGN and Star-Formation Activity
Marc Sarzi, Elias Brinks, Johan Knapen (IAC, Tenerife)
The finding of a striking relation between the mass of the central supermassive black hole (SMBH) and that of the spheroidal component in galaxies demonstrates that the growth of black holes and the star formation history of galaxies must be tightly linked. However, the triggering of either the nuclear activity or the central star formation is not yet fully understood. This project will combine, for a considerable number of early-type galaxies, the unprecedented sensitivity, spectral and spatial resolution, and field-of-view of the radio continuum imaging from the LeMMINGs (Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies) survey with integral-field spectroscopic data from SAURON, which will provide access to the gas kinematics and stellar dynamics. Based on this unique combination of radio and optical data, the PhD student will be able to assess not only the extent and interplay of both the star formation and the nuclear activity in early-type galaxies, but also measure the gas inflow towards the centre and the stability of the gas against gravitational collapse, which in turn will help understanding what triggers both the growth of SMBHs and the star formation around them.
The starburst-AGN connection at high redshift
Kristen Coppin, Jason Stevens, James Geach
Two key processes that are crucial to the formation of massive galaxies are the build-up of stellar mass and the growth of a central supermassive black hole (SMBH). In the distant Universe, the progenitors of massive galaxies can be identified as bolometrically luminous galaxies such as those selected in the submillimetre bands (SMGs). SMGs are powered by starbursts and active galactic nuclei (AGN), and are a phase of rapid galaxy growth. It is widely believed that luminous starbursts and AGN are triggered by galaxy interactions and merging. An evolutionary model is emerging whereby every SMG would host an AGN, which would eventually grow a central SMBH strong enough to blow off all of the gas and dust leaving an optically luminous quasar. A meaningful way to probe this picture is to select galaxies at key phases in this sequence and study their properties in detail, such as their morphologies, luminosities, star formation rates, black hole masses, and gas masses. This project will involve studying the starburst AGN connection at high redshift using a multiwavelength approach and comparing the results to simulations, including the analysis of data from some of the world's top facilities including HST, ALMA, Spitzer, Herschel, and the JCMT.
PNe as probes of the stellar population of Early-type Galaxies
Marc Sarzi, Ralf Napiwotzki
In extra-galactic astronomy, Planetary Nebulae (PNe) are regarded mostly as useful indicators for the distance or dark-matter content of their galactic hosts. Yet, PNe can also be used as probes of their parent stellar population, in particular by studying the shape and normalisation of their luminosity function. Today, most known PNe have been found in the outskirts of their hosts, because they were observed with narrow-band imaging or slit-less spectroscopy that find it hard to detect PNe against a stellar background. Yet, it is in the central regions of galaxies that the age and metallicity of stellar populations show the largest variations within a single galaxy or between different objects. Integral-field spectroscopy can overcome such instrumental limitation since it allows for the careful subtraction of the galaxy spectrum, as demonstrated by Sarzi et al. (2010) and Pastorello, Sarzi et al (2012) for our closest companions M31 and M32, respectively. This PhD project will extend these first exercises to the entire SAURON sample of 48 early-type galaxies (de Zeeuw et al. 2001). Since the stellar population properties of these galaxies are already known (Kuntschner et al. 2010) this work will be the first systematic investigation of the link between the PNe content and the stellar populations of early-type galaxies.
Galaxy growth in the nodes of the cosmic web
Jason Stevens, James Geach
Galaxies form and evolve within a cosmic web of large scale structure, which is itself evolving due to gravity. It is thought that galaxies growing within the densest parts of the web - those regions destined to become rich clusters of galaxies today - undergo an 'accelerated' evolution (e.g. in stellar mass assembly and black hole growth) compared to galaxies in average density regions of the Universe. Identifying the mechanisms governing this accelerated evolution is a key part of our model of the formation of galaxies, but the role of environment on galaxy evolution at high-redshift remains poorly understood. This project will involve multi-wavelength (x-ray/optical/infrared/submm/radio) studies of galaxies within several known high-redshift 'proto-clusters'. The goal will be to assemble an evolutionary picture of galaxy growth within these remarkable environments. The student will work with a wide range of observational data, including Chandra (x-ray), HST (optical) and SCUBA-2 (submm) in order to constrain the star formation and black hole growth histories and structural evolution of galaxies. An additional component will be to use the latest deep wide-field imaging surveys to identify new proto-cluster candidates with which to boost the sample.
Galaxy evolution in the first half of cosmic time
Sugata Kaviraj, Elias Brinks, Kristen Coppin, Jim Geach
Our understanding of how primordial galaxies emerged in the early Universe remains scant. This project will combine state-of-the-art data from CANDELS (the largest Hubble Space Telescope survey to date) with the Galaxy Zooproject (which uses 600,000+ members of the general public to classify galaxy morphologies in large datasets) to probe some of the compelling open questions in observational cosmology today e.g. at what epochs was the Hubble morphological sequence established? How and when were the early spheroidal galaxies formed? What was the relative role of major mergers and secular processes (e.g. cold flows) in driving star formation and black hole growth in the early Universe? How well do our current galaxy formation models capture the emergence of massive galaxies at high redshift?
The overall aim is to construct an unprecedentedly detailed picture of galaxy evolution in the first half of cosmic time and stimulate the exploitation of next-generation instruments like the James Webb Space Telescope (successor to the Hubble) and the European-Extremely Large Telescope (E-ELT)
Young stars in ancient galaxies
Sugata Kaviraj, Elias Brinks, Marc Sarzi
Early-type galaxies dominate the stellar mass density in the local Universe, making them unique probes of galaxy evolution over cosmic time. While they were traditionally considered to be purely old and passively-evolving,new ultraviolet (UV) studies have revolutionised our understanding of these key systems, revealing widespread recent star formation over the last 8 billion years, driven by Œminor¹ mergers between early-types and small gas-rich dwarfs. Nevertheless, the details of the minor-merger process that drives this star formation remain very poorly understood.
Using state-of-the-art UV-optical data from a proprietary Hubble Space Telescope programme, this project will perform a high-resolution study of star formation in early-type galaxies, spatially mapping the properties of the young stars, age-dating star clusters, quantifying the star formation law and reconstructing the properties of the progenitor systems via comparison to detailed numerical simulations. The overall aim will be to significantly enhance our understanding of the poorly-understood minor merger process and develop tools for the exploitation of future instruments like the European-Extremely Large Telescope, which will make high-resolution imaging routine at the end of this decade.